What you could do:
Get a Resistor of 0.01 Ohm.
Place it inbetween the cable you want to check.
Use a multimeter and meassure over (parallel) the resistor.
You will expect less than 1 Volt DC.
The math is here: 1Amp ~ 0.01Volt
So if you have a load of 40Amps the Voltage over the Resistor is 0.40Volt.
The Amps and Volts will heat up the Resistor by their product:
40A x 0.4V = 16Watt or only 1.6W with 4A
If you can't find a 0.01Ohm resistor, the math is like this:
Volts = Amps x Ohms or Amps = Volts / Ohms
Remember that the Volts are also the value the Voltage drops for the devices behind. So if the resistor ist to high, you loose a lot of Voltage and the Resistor gets hot (Watts = Volts x Amps).
You can built your own Resistor with a copperwire.
12 AWG = 0,00159786 Ohm/feet
10 AWG = 0,00100386 Ohm/feet
8 AWG = 0,00063261 Ohm/feet
Have fun.
carl